Multiple master functions are today being commonly integrated onto a single system chip. When initially defining an architecture for the integration of multiple discrete components onto a single chip, access to external devices can be an issue. For example, an MPEG video decoder system often employs external memory for various data areas, or buffers such as frame buffers. This external memory is conventionally implemented using either DRAM or SDRAM technology.
Two approaches are typical in the art for accessing off-chip devices. In a first approach, each on-chip functional unit is given access to the needed external device(s) through a data bus dedicated to that particular unit. Although locally efficient for accessing the external device, globally within the integrated system this approach is less than optimal. For example, although each function will have complete access to its own external memory area, there is no shared access between functions of the integrated system. Thus, transferring data from one memory area to another memory area of the system is often needed. This obviously increases the number of data transfers and can degrade performance of the overall system, i.e., compared with a shared memory system.
Another approach is to employ a single common bus within the integrated system which allows one or more functional units of the system to communicate to external devices through a single port. Although allowing the sharing of devices, one difficulty with this approach concerns controlling access to content or other sensitive data in the integrated system. For example, when using a large common memory pool in an integrated design, it becomes difficult to prevent unauthorized access to protected memory spaces, such as compressed data supplied by a transport demultiplexer to a decoder of a set-top box. This is especially true for a system where the programming interface is open and outside development is encouraged. Each of the functional masters should be able to access the memory space and it is not possible to differentiate whether an access is from a trusted master or an outside request, e.g., coming through an untrusted or open master.
In addition, when working with a system-on-chip design with multiple functional masters using shared memory, it is desirable to provide a mechanism for protecting the data from unauthorized access, particularly when the data comprises the device's system programming code. Encryption can be used to protect code and data structures stored in a system, but the encrypted values could be modified, either intentionally or unintentionally, while the values are in unsecured or open storage. When an integrated system decrypts corrupted values, it will yield an incorrect result. While this prevents unauthorized control of the system, it can lead to operational problems since the incorrect result could be a random value and cause unwanted actions, such as machine checks, incorrect data, etc.
In view of the above, a need exists in the art for an enhanced access control approach for an integrated system. More particularly, a need exists for an access control function which includes a means of authenticating, or verifying, the integrity of code or data retrieved from storage.